linux-kernel - Re: [PATCH net-next 3/4] hv_netvsc: reset net_device_ctx->nvdev with rcu_assign

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Date:   Tue, 31 Oct 2017 07:09:58 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        devel@...uxdriverproject.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH net-next 3/4] hv_netvsc: reset net_device_ctx->nvdev
 with rcu_assign_pointer()

On Tue, 2017-10-31 at 14:42 +0100, Vitaly Kuznetsov wrote:
> RCU_INIT_POINTER() is not suitable here as it doesn't give us ordering
> guarantees (see the comment in rcupdate.h). This is also not a hotpath.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  drivers/net/hyperv/netvsc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
> index bfc79698b8f4..12efb3e34775 100644
> --- a/drivers/net/hyperv/netvsc.c
> +++ b/drivers/net/hyperv/netvsc.c
> @@ -560,7 +560,7 @@ void netvsc_device_remove(struct hv_device *device)
>  
>  	netvsc_revoke_buf(device, net_device);
>  
> -	RCU_INIT_POINTER(net_device_ctx->nvdev, NULL);
> +	rcu_assign_pointer(net_device_ctx->nvdev, NULL);

I see no point for this patch.

Setting a NULL pointer needs no barrier at all.